The present invention relates generally to improved diaphragm-type electrolytic cells having dimensionally stable metal anodes, foraminous steel cathodes and asbestos-free, polymeric microporous separators. The asbestos-free diaphragm cells of the subject invention are suitably used as chlor-alkali cells and other liquid-liquid processing apparatuses like fuel cells, osmotic cells, diffusional cells, and the like. More particularly, the invention disclosed and claimed herein is concerned with more efficient operating electrolytic cells wherein a polymeric microporous separator is in supportive contact with an electroconductive, protective foraminous cathode, said cathode being positioned between the separator and the cell's steel cathode. The iron-free electroconductive surface of the protective cathode eliminates, at least partially, plugging and loss of porosity of the separator. Without the protective cathode of the present invention, soluble corrosion products from the primary steel cathode developing during cell shut-down enter the separator to form insoluble deposits. During cell start-up the deposit build-up adversely affects cell performance, power consumption and product purity. Furthermore, because the iron oxide deposits are not easily removed the life expectancy of an affected microporous separator is severely shortened.
Diaphragm cells have long been used for the manufacture of chlorine and caustic soda. In such cells, anolyte and catholyte liquors have been previously separated by a diaphragm of deposited asbestos fibers, usually on a steel mesh cathode structure. In recent years, however, such diaphragms in some instances have been replaced by ion-permeable membranes or porous separators. Of the porous separators, those which are microporous and made, for example, from polytetrafluoroethylene (PTFE) are gaining increased favor, primarily for reasons of environmental safety, lower electrical power consumption and overall lower cell maintenance costs.
Because polymeric microporous separators are usually made in the form of sheets and not deposited onto a cell cathode like an asbestos diaphragm various methods have been proposed for cell retrofitting. One satisfactory method is disclosed in U.S. Pat. No. 4,283,264, which teaches a porous PTFE material retrofitted onto a chlor-alkali cell cathode wherein a plurality of open-ended tubular panels of a height greater than the cell cathode are equipped with polymeric flanges. The anode compartments of the cell are sealed off from the cathode compartment by bonding halves of two adjacent separator tubes. U.S. Pat. No. 4,076,571 teaches another method whereby separator envelopes are formed by heat sealing edges together; slipped over an electrode followed by closing the envelopes with clamping members.
Regardless of the method employed in the installation of polymeric separators, greater narrowing of the anode-cathode gap tolerances and cell geometry invariably requires separator panels to be in direct contact with the active surfaces of the cathode. As a result of this direct contact between polymeric separator and steel cathode, current blockage takes place and corrosion of the cathode occurs at points of contact. During periods of cell shut-down, soluble iron oxide corrosion products from the steel cathode collect in the separator pores forming insoluble iron oxide deposits after cell start-up which in-turn migrate to the outer surface of the separator. The deposits have the negative effect of causing at least a partial plugging of the separator and loss of porosity, inducing elevated voltages and higher power consumption. In addition, the separator deposits create active cathode sites where hydrogen can be evolved in the anolyte contaminating the halogen gas being formed. A build-up of deposits will also create excess anolyte head heights requiring early replacement of the separator.
Heretofore, various devices, including cell diaphragm additives and coatings have been suggested as means for improving cell performance. For example, British patent specification No. 1,336,225 and U.S. Pat. No. 3,989,615 and in particular the former disclose a chlor-alkali cell comprising a fibrous diaphragm and a supporting net disposed between the diaphragm and cathode. The supporting net is fabricated from stainless steel, titanium or iron. However, titanium metal is subject to hydrogen embrittlement and will dissolve in the catholyte while iron and stainless steel are capable of forming insoluble metal oxides in the diaphragm thereby reducing separator porosity. U.S. Pat. Nos. 2,944,956 and 3,344,053 and in particular the latter suggests placement of a secondary screen adjacent to the diaphragm facing the cathode. However, the screen has an outer polymeric coating rendering it electrically non-conductive and hydrophobic to cell electrolyte. U.S. Pat. No. 4,165,271 describes a diaphragm comprised of a support fabric, including PTFE which is impregnated with a gel-forming silica material and a non-continuous electroconductive surface coating of nickel, nickel alloys, platinum group metals and their alloys. The application of non-continuous coatings on separator surfaces have shorter life expectancies, especially during recurring cell shut-down periods. Accordingly, there is a need for an improved secondary cathode to be used in conjunction with a polymeric microporous separator equipped electrolytic cell.
It has now been discovered that an independent foraminous protective cathode placed as a barrier to direct contact between the primary steel cathode and a polymeric microporous separator of an electrolytic cell will greatly extend the useful life expectancy of the separator. The protective cathode, in the form of a mesh or screen has a stable, continuous electroconductive metallic surface which is also hydrophillic to cell contents.
Accordingly, it is a principal object of the present invention to provide a method for extending the useful life expectancy of asbestos-free, microporous separators used in electrolytic cells.
A further object of the immediate invention is a solution to the problem of corrosion products from the primary steel cathode of an electrolytic cell depositing and plugging microporous separators as a result of cell shutdown.
A still further object of the present invention is the fabrication of a protective cathode screen having an electroconductive metallic surface which is more stable than steel to the corrosive environment of a chlor-alkali cell.